1. Field of the Invention
The invention relates to a compound having a hydrocoumarin skeleton, a liquid crystal composition, and a liquid crystal display device.
2. Related Art
A liquid crystal display device is classified, depending on the display mode thereof, into such modes as DS (dynamic scattering), TN (twisted nematic), GH (guest host), STN (super twisted nematic), IPS (in-plane switching), VA (vertical alignment) and OCB (optically compensated bend). A liquid crystal composition contained in the liquid crystal display devices desirably has some or all of the following common characteristics in all the display modes.    (1) The composition is stable to external environmental factors, such as water, air, heat and light.    (2) The composition exhibits a liquid crystal phase in a wide temperature range centering around room temperature.    (3) The composition has a small viscosity.    (4) The composition can decrease a driving voltage upon driving the display device.    (5) The composition has an optimum dielectric anisotropy (Δ∈).    (6) The composition has an optimum optical anisotropy (Δn).
However, such a liquid crystal compound has not yet been found that satisfies all of characteristics (1) to (6) by a sole compound. Therefore, a liquid crystal composition is often obtained by mixing several kinds (e.g., a couple dozen kinds) of liquid crystal compounds. Accordingly, the liquid crystal compounds used as components of the composition necessarily have good compatibility with each other. A liquid crystal display device capable of being used in various environments, such as a very low temperature, has been demanded in recent years, and liquid crystal compounds exhibiting good compatibility at a very low temperature are thus also demanded.
In recent years, among the aforementioned display modes, such modes as IPS, VA and OCB are receiving attention as a display mode capable of overcoming a narrow viewing angle of a liquid crystal display device, which is the biggest problem of a liquid crystal display device. In particular, liquid crystal display devices of the VA mode and the IPS mode among these modes are being developed earnestly since it has excellent response in addition to the wide viewing angle, and is capable of providing high-contrast display. The characteristics of the liquid crystal composition used in the liquid crystal display device of these modes reside in a negative dielectric anisotropy. It has been known that a liquid crystal composition having a large negative dielectric anisotropy can decrease the driving voltage of a liquid crystal display device containing the liquid crystal composition (as described in M. F. Leslie, Mol. Cryst. Liq. Cryst., vol. 12, p. 57 (1970)). Accordingly, liquid crystal compounds as the constitutional components of the liquid crystal composition are also demanded to have a larger negative dielectric anisotropy.
As a component of a liquid crystal composition having a negative dielectric anisotropy, various liquid crystal compounds where hydrogen at a lateral position of a benzene ring is replaced by fluorine have been investigated (as described, for example, in Japanese Patent No. 2,811,342 and JP H2-4725 A/1990). The following compound (c) (wherein R and R′ are alkyls) has been reported, for example.

However, the compounds exemplified by the compound (c) have a negative dielectric anisotropy, but the extent thereof is not necessarily large in some cases, and therefore, the compounds are not sufficient in some cases for decreasing the driving voltage of the liquid crystal display devices of the VA mode and the IPS mode.
As a liquid crystal compound having a large negative dielectric anisotropy, the compound (d) has also been reported (as described in JP S59-10557 A/1984). The compound (d) (wherein R and R′ are alkyls) has a considerably large negative dielectric anisotropy, but is not necessarily sufficient in chemical and physical stability.

In addition to the aforementioned compounds, compounds having hydrocoumarin have also been reported (JP H3-240783 A/1991, JP H4-210687 A/1992 and JP 2004-292774 A/2004).
For example, JP H3-240783 A/1991 and JP H4-210687 A/1992 disclose compounds having a hydrocoumarin skeleton, but the purposes of the compounds are restricted to a ferroelectric liquid crystal. The compounds specifically disclosed in JP H3-240783 A/1991 and JP H4-210687 A/1992 have a carboxyl group having branched alkyl or a long-chain alkyl group at an end thereof. However, these compounds are not suitable for use as a nematic liquid crystal due to a high viscosity and a strong smectic nature thereof.
JP 2004-292774 A/2004 discloses a general formula containing a skeleton that is apparently similar to the compound of the invention. However, the compound that has been clarified for the structural formula and properties thereof is limited to one having one benzene ring and one lactone ring, and a liquid crystal composition containing the compound is liable to have a low clearing point. Accordingly, the compound has room for improvement upon using as a nematic liquid crystal.